Fluidized catalytic cracking of heavy petroleum fractions is one of the major refining methods to convert crude or partially refined petroleum oil to useful products such as gasoline and heating oils. In fluidized catalytic cracking, high molecular weight hydrocarbon liquids are contacted with hot, finely divided solid catalyst particles in a catalytic reactor. The reactor is typically in the form of a riser pipe, and the contact time of the hydrocarbon feed is on the order of a few seconds. This short contact time is necessary to optimize generation of gasoline and middle distillate fractions and to minimize economically undesirable end products, such as methane and carbon. Reaction of the feed material generates a large volume of gaseous hydrocarbons. These hydrocarbons in admixture with the catalyst flow out of the riser pipe into a separator or disengaging section of the fluid catalytic cracking unit (FCCU). The spent catalyst is separated and passed downwardly through a stripper section for return to a regenerator. Fluidizing steam is typically introduced to flow up through the down-flowing catalyst to assist in stripping hydrocarbon vapor from the spent catalyst. The desired product, in the form of hydrocarbon gas or vapor, is recovered overhead, typically through one or more cyclone separators, and is piped to a distillation column.
One problem with the injection of a hydrocarbon feed into the catalytic reactor is that if the feed is not sufficiently atomized and does not directly contact catalyst upon injection into the reactor, then thermal cracking may occur instead of catalytic cracking. This results in the generation of the undesirable end products of methane and coke, rather than the desired middle distillate hydrocarbons. Prolonged contact of the hydrocarbon feed with catalyst after discharge into the stripper section of the FCCU may also result in overcracking or further thermal cracking.
There is considerable evidence that the atomization and subsequent vaporization of the liquid feed has important effects in fluid catalytic cracking operations. Generally the most desirable reactions occur in the vapor phase and require extremely rapid movement of reactants to and from active catalytic sites. In addition, the time required for vaporization of feed droplets reduces the time available for the desired catalytic reactions. Furthermore, liquid wetting of the catalyst tends to reduce the surface area which is available to catalyze hydrocarbon reactions, and results in increased "coke" due to adsorption of heavy fractions present in the feed or formed by polymerization. Excess coke is undesirable because the process duties of the stripper and regenerator are increased as a result. In addition, coke can deposit on the surfaces of the equipment involved.